Method of preparing iron-based components

ABSTRACT

The present invention concerns a process for the preparation of high density green compacts comprising the steps of providing an iron-based powder essentially free from fine particles; optionally mixing said powder with graphite and other additives; uniaxially compacting the powder in a die at a compaction pressure of at least about 800 MPa and ejecting the green body. The invention also concerns the powder used in the method.

FIELD OF THE INVENTION

[0001] The present invention relates to metal powder compositions usefulwithin the powder metallurgical industry. More specifically theinvention concerns a method for the preparation of components havinghigh density by using these compositions.

[0002] There are several advantages by using powder metallurgicalmethods for producing structural parts compared with conventionalmatching processes of full dense steel. Thus, the energy consumption ismuch lower and the material utilisation is much higher. Anotherimportant factor in favour of the powder metallurgical route is thatcomponents with net shape or near net shape can be produced directlyafter the sintering process without costly shaping processes such asturning, milling, boring or grinding. However, normally a full densesteel material has superior mechanical properties compared with PMcomponents. This is mainly due to the occurrence of porosity in the PMcomponents. Therefore, the strive has been to increase the density of PMcomponents in order to reach values as close as possible to the densityvalue of a full dense steel.

[0003] Among the methods used in order to reach higher density of PMcomponents the powder forging process has the advantage that full densecomponents may be obtained. The process is however costly and isutilised mainly for mass production of heavier components, such asconnection rods. Full dense materials can also be obtained by elevatedpressures at high temperatures, such as in hot isostatic pressing, HIP,but also this method is costly.

[0004] By using warm compaction, a process where the compaction isperformed at an elevated temperature, typically at 120 to 250° C., thedensity can be increased with about 0.2 g/cm³, which results in aconsiderable improvement of the mechanical properties. A disadvantage ishowever that the warm compaction method involves additional investmentand processing. Other processes, such as double pressing, doublesintering, sintering at elevated temperatures etc, may further increasethe density. Also these methods will add farther production costs hencereducing the overall cost effectiveness.

[0005] In order to expand the market for powder metallurgical componentsand utilise the advantages with the powder metallurgical technique thereis thus a need for a simple, less expensive method of achieving highdensity compacts with improved static and dynamic mechanical strength.

SUMMARY OF THE INVENTION

[0006] It has now been found that high density components can beobtained by using high compaction pressures in combination with coarsepowders. In view of the general knowledge, that conventionally usedpowders, i.e. powders including fine particles, cannot be compacted tohigh densities without problems with e.g. damaged or deterioratedsurfaces of the compacts this finding is quite unexpected. Specifically,the method according to the present invention includes the steps ofproviding an iron-based powder essentially free from fine particles;optionally mixing said powder with graphite and other additives;uniaxially compacting the powder in a die at high pressure and ejectingthe green body, which may subsequently be sintered.

DETAILED DESCRIPTION OF THE. INVENTION

[0007] The term “high density” is intended to mean compacts having adensity of about at least 7.3 g/cm³. Components having lower densitiescan of course also be produced but are believed to be of less interest.

[0008] The iron-based powder according to the present invention includespure iron powder such as atomised iron powder, sponge iron powder,reduced iron powder; partially diffusion-alloyed steel powder; andcompletely alloyed steel powder. The partially diffusion-alloyed steelpowder is preferably a steel powder alloyed partially with one or moreof Cu, Ni, and Mo. The completely alloyed steel powder is preferably asteel powder alloyed with Mn, Cu, Ni, Cr, Mo, V, Co, W, Nb, Ti, Al, P, Sand B. Also stainless steel powders are of interest.

[0009] As regards the particle shape it is preferred that the particleshave an irregular form as is obtained by water atomisation. Also spongeiron powders having irregularly shaped particles may be of interest.

[0010] A critical feature of the invention is that the powder used havecoarse particles i.e. the powder is essentially without fine particles.The term “essentially without fine particles” is intended to mean thatless than about 5% of the powder particles have a size below 45 μm asmeasured by the method described in SS-EN 24 497. So far the mostinteresting results have been achieved with powders essentiallyconsisting of particles above about 106 μm and particularly above about212 μm. The term “essentially consists” is intended to mean that atleast 50%, preferably at least 60%, and most preferably at least 70% ofthe particles have a particle size above 106 and 212 μm, respectively.The maximum particle size may be about 2 mm. The particle sizedistribution for iron-based powders used at PM manufacturing is normallydistributed with a gaussian distribution with a average particlediameter in the region of 30 to 100 μm and about 10-30% less than 45 μm.Iron based powders essentially free from fine particles may be obtainedby removing the finer fractions of the powder or by manufacturing apowder having the desired particle size distribution.

[0011] The influence of particle size distribution and the influence ofparticle shape on the compaction properties and properties of thecompacted body have been subjected to intense studies. Thus the U.S.Pat. No. 5,594,186 reveals a method of producing PM components with adensity higher than 95% of theoretical density by utilisingsubstantially linear, acicular metal particles having a triangular crosssection. Such particles are suitably produced by a machining or millingprocess.

[0012] Powders having coarse particles are also used for the manufactureof soft magnetic components. Thus the U.S. Pat. No. 6,309,748 disclosesa ferromagnetic powder, the particles of which have a diameter sizebetween 40 and 600 μm. In contrast to iron based powder particlesaccording to the present invention, these powder particles are providedwith a coating.

[0013] In the U.S. Pat. No. 4,190,441 a powder composition forproduction of sintered soft magnetic components is disclosed. In thispatent the iron powder includes particles with less than 5% exceeding417 μm, and less than about 20% of the powder particles have a size lessthan 147 μm. This patent teaches that, because of the very low contentof particles less than 147 μm, the mechanical properties of componentsmanufactured from this coarse, highly pure powder are very low.Furthermore the patent teaches that if higher strength is desired, it isnot possible to increase the content of particles having a size lessthan 147 μm without simultaneously deteriorating the soft magneticproperties. Therefore this powder is mixed with specific amounts offerrophosphorus. Graphite which may be used in the compositionsaccording to the present invention is not mentioned in this patent andbesides the presence of graphite would deteriorate the magneticproperties.

[0014] Powder mixtures including coarse particles are also disclosed inthe U.S. Pat. No. 5,225,459 (E 554 009) which also concerns powdermixtures for the preparation of soft magnetic components. Nor do thesepowder mixtures include graphite.

[0015] Within the field of powder forcing it is furthermore known thatpre-alloyed iron-based powders with coarse particles can be used. TheU.S. Pat. No. 3,901,661 discloses such powders. This patent disclosesthat a lubricant may be included and specifically that the amount oflubricant should be 1% by weight (example 1). If the powders accordingto the present invention were mixed with such a high amount of lubricantit would however not be possible to achieve the high densities.

[0016] In order to obtain compacts having satisfactory mechanicalsintered properties of the sintered part according to the presentinvention it is necessary to add certain amounts of graphite to thepowder mixture to be compacted. Thus graphite in amounts between 0.1-1,preferably 0.2-1.0 and most preferably 0.2-0.8% by weight of the totalmixture to be compacted could be added before the compaction.

[0017] Other additives may be added to the iron-based powder beforecompaction such as alloying elements comprising Mn, Cu, Ni, Cr, Mo, V,Co, W, Nb, Ti, Al, P, S, and B. These alloying elements may be added inamounts up to 10% by weight. Further additives are machinabilityenhancing compounds, hard phase material and flow agents.

[0018] The iron-base powder may also be combined with a lubricant beforeit is transferred to the die (internal lubrication). The lubricant isadded to minimize friction between the metal power particles and betweenthe particles and the die during a compaction, or pressing, step.Examples of suitable lubricants are e.g. stearates, waxes, fatty acidsand derivatives thereof, oligomers, polymers and other organicsubstances with lubricating effect. The lubricants are preferably addedin the form of particles but may also be bonded and/or coated to theparticles. According to the present invention the amount of lubricantadded to the iron-based powder may vary between 0.05 and 0.6%,preferably between 0.1-0.5% by weight of the mixture.

[0019] The method according to the invention may also be performed withthe use of external lubrication (die wall lubrication) where the wallsof the die are provided with a lubricant before the compaction isperformed. A combination of external and internal lubrication may alsobe used.

[0020] The term “at high compaction pressure” is intended to mean atpressures of about at least 800 MPa. More interesting results areobtained with higher pressures such as pressures above 900, preferablyabove 1000, more preferably above 1100 MPa.

[0021] Conventional compaction at high pressures, i.e. pressures aboveabout 800 MPa with conventionally used powders including finerparticles, in admixture with low amounts of lubricants (less than 0.6%by weight) are generally considered unsuitable due to the high forcesrequired in order to eject the compacts from the die, the accompanyinghigh wear of the die and the fact that the surfaces of the componentstend to be less shiny or deteriorated. By using the powders according tothe present invention it has unexpectedly been found that the ejectionforce is reduced at high pressures, about 1000 MPa, and that componentshaving acceptable or even perfect surfaces may be obtained also when diewall lubrication is not used.

[0022] The compaction may be performed with standard equipment, whichmeans that the new method may be performed without expensiveinvestments. The compaction is performed uniaxially in a single step atambient or elevated temperature. Alternatively the compaction may, beperformed with the aid of a percussion machine (Model HYP 35-4 fromHydropulsor) as described in patent publication. WO 02/38315.

[0023] The sintering may be performed at temperatures normally usedwithin the PM field, e.g. at standard temperature between 1080 and 1160C° C. or at higher temperatures above 1160° C. and in conventionally usedatmospheres.

[0024] Other treatments of the green or sintered component may as wellbe applied, such as machining case hardening, surface densification orother methods used in PM technology.

[0025] In brief the advantages obtained by using the method according tothe present invention are that high density green compacts can be costeffectively produced. The new method also permits production of highercomponents which are difficult to produce by using the conventionaltechnique. Additionally standard compaction equipment can be used forproducing high density compacts having acceptable or even perfectsurface finish.

[0026] Examples of products which suitably can be manufactured by thenew method are connecting rods, gears and other structural partssubjected to high loads. By using stainless steel powders flanges are ofspecial interest.

[0027] The invention is further illustrated by the following examples.

EXAMPLE 1

[0028] Two different iron-based powder compositions according to thepresent invention were compared with a standard iron-based powdercomposition. All three compositions were produced with Astaloy Moavailable from Höganäs AB, Sweden. 0.2% by weight of graphite and 0.4%by weight of a lubricant (Kenolube™) were added to the compositions. Inone of the iron-based powder compositions according to the invention,particles of the Astaloy Mo with a diameter less than 45 μm were removedand in the other composition according to the invention particles ofAstaloy Mo less than 212 μm were removed. The compaction was performedat ambient temperature and in standard equipment. As can be seen fromFIG. 1-1 a clear density increase at all compaction pressures isobtained with the powder having a particle size above 212 μm.

[0029]FIG. 1-2 shows that in order to obtain components withoutdeteriorated surfaces the most important factor is the reduction orelimination of the smallest particles, i.e. particles below 45 μm.Furthermore from this figure it can be seen that the force needed forejection of the compacts produced by the iron based powder compositionwithout particles less than 212 μm was considerably reduced comparedwith the ejection force needed for compacts produced from the standardiron-based powder composition having about 20% of the particles lessthan 45 μn. The ejection force needed for compacts produced from theiron-based powder composition according to the invention withoutparticles less than 45 μm is also reduced in comparison with thestandard powder.

[0030] A noticeable phenomenon is that the ejection force for compactsproduced according to the present invention decreases with theincreasing ejection pressure whereas the opposite is valid for thestandard composition.

[0031] It was also observed that the compacts obtained when the standardpowder was compacted at a pressure above 700 MPa had deterioratedsurfaces and were accordingly not acceptable. The compacts, which wereobtained when the powder essentially without particles less than 45 μmwas compacted at a pressure above 700 MPa, had a less shiny surfacewinch at least under certain circumstances is acceptable.

EXAMPLE 2

[0032] Example 1 was repeated but as lubricant 0.5% of EBS (ethylenebisstearamide) was used and the compaction was performed with the aid ofa percussion machine (Model HYP 35-4 from Hydropulsor, Sweden)

[0033] From FIGS. 2-1 and 2-2, respectively, it can be noticed thathigher green densities and lower ejection forces were obtained with thepowder composition according to the invention compared with the powdercomposition with the standard, powder. It can also be noticed thatcomponents produced from the standard powder had deteriorated surfacesat all compaction pressures.

1. Process for the preparation of high density green compacts comprisingthe following steps: providing an iron or iron-based powder wherein lessthan about 5% of the iron-based powder particles have a size below 45μm; optionally mixing said powder with graphite and other additives;uniaxially compacting the powder in a die at a compaction pressure of atleast about 800 MPa and ejecting the green body from the die.
 2. Processaccording to claim 1 wherein the compaction is performed in a singlestep.
 3. Process according to claim 1 or 2, wherein at least 50%,preferably at least 60% and most preferably at least 70% of theiron-based powder consists of particles having a particle size aboveabout 106 μm.
 4. Process according to any one of the claims 1-3, whereinat least 50%, preferably at least 60% and most preferably at least 70%of the iron-based powder consists of particles having a particle sizeabove about 212 μm.
 5. Process according to claim 4, wherein the maximumparticle size is about 2 mm.
 6. Process according to any of claims 2-5,wherein the graphite is present in an amount of 0.1-1.0%.
 7. Processaccording to any of claims 1-6, wherein the iron-based powder iscombined with a lubricant in an amount between 0.05 and 0.6% by weightbefore compaction.
 8. Process according to any of claims 1-6, whereinthe compaction is performed in a lubricated die.
 9. Process according toany of claims 7-8, wherein the compaction is performed by using acombination of internal and external lubrication.
 10. Process accordingto any of claims 1-9 wherein the additives are selected from the groupconsisting of alloying elements such as Mn, Cu, Ni, Cr, Mo, V, Co, W,Nb, Ti, Al, P, S and B machinability enhancing agents, hard phasematerials and flow agents.
 11. Process according to any of claims 1-10,wherein the compaction is performed at a pressure of at least 900 MPa,more preferably at least 1000 and most preferably above 1100 MPa. 12.Process according to any of claims 1-11, wherein the compaction isperformed at ambient temperature.
 13. Process according to any of claims1-11, wherein the compaction is performed at elevated temperature 14.Process according to any of claims 1-13 for preparing sintered productssaid process further including a single sintering step at a temperatureabove 1100° C.
 15. Powder composition comprising an iron or iron-basedpowder wherein less than about 5% of the powder particles have a sizebelow 45 μm; and 0.1-1.0% by weight of graphite.
 16. Powder compositionaccording to claim 15 further including 0.05-0.6% by weight of alubricant.
 17. Composition according to claim 15 or 16, wherein at least50%, preferably at least 60% and most preferably at least 70% of theiron-based powder have a particle size above about 106 μm. 18.Composition according to claim 17, wherein at least 50% of theiron-based powder particles have a particle size above about 212 μm. 19.Compositon according to any one of the claims 15-18 further includingadditives selected from the group consisting of alloying elements suchas Mn, Cu, Ni, Cr, Mo, V, Co, W, Nb, Ti, Al, P, S and B machinabilityenhancing agents, hard phase materials and flow agents